Optical waveguide modulators and switches which have been operative objectives comparable to those of the present invention have been previously described in several U.S. patents including U.S. Pat. No. 3,408,131 for an "Apparatus for Coupling and Amplifying Light Between Optical Fibers," issued to Narinder S. Kapany, Oct. 29, 1968, and U.S. Pat. No. 3,589,794 for "Optical Circuits," issued to Enrique A. J. Marcatili, issued June 29, 1971. This type of prior art developments were, however, characterized by an electrical drive signal applied in phase along the length of the optical waveguide or alternatively so as to form a standing wave along the length of the optical waveguide. Moreover, the electrodes that were employed in this prior art type of system did not form an electrical transmission line. Accordingly, the length of such structures was limited in efficient bandwidth embodiments to a fraction of the electrical drive signal wavelength involved, as principally determined by the properties of the device material and the maximum frequency of the electrical drive signal.
In other types of prior art the limitation on the length of the modulator was overcome by travelling wave modulators of the type described by W. W. Rigrod and I. P. Kaminow in an article entitled "Wideband Microwave Light Modulation," published in the Proceedings of the IEEE, 1963.
In this type of prior art the phase velocities of the electrical and the optical energies were matched along one axial direction of propagation. However, the light propagated in an unbounded media or through a sequence of lenses in the plane transverse to the direction of propagation and the electrical energy occupied a region whose dimensions were several orders of magnitude larger than the wavelength of the light energy.
The electrical energy was guided co-linearly with the optical energy by a metallic walled waveguide or by two metallic surfaces which serve to reflect the microwave energy back and forth across the beam of light. In the dimensions transverse to the direction of propagation of the light the electrical energy was confined to a region whose linear dimensions were of the order of one wavelength of the electrical energy involved.
The magnitude of the change in the optical wavelength induced by the electrical energy was proportional to the magnitude of the electrical field strength of the electrical energy throughout the region in which the magnitude of the optical energy is significant. The electrical power required to induce a given optical wavelength change was proportional to the product of the electrical field strength and the transverse dimension over which the electrical field strength is significant.
There is, therefore, a need for a modulator which inherently requires lateral dimensions much smaller than comparable dimensions in prior art devices so as to afford a correlated reduction in the electrical power required for the operation of the modulator in a practical device. In addition it is highly desirable that the electrical and optical energies be confined by structures that enable the electrical and optical energies to propagate co-linearly at substantially the same phase velocity in order that broad bandwidth operation to be readily attainable.
Accordingly, there is a need for an optical waveguide modulator and switch in which the length of the device is not limited in the manner that was characteristic of functionally comparable prior art devices. Moreover, it is highly desirable that in addition to the efficient broad bandwidth operation, the optical waveguide modulator and switch require a minimum magnitude of electrical drive signal for an acceptable and efficient level of operation.